1. Field of the Invention
The present invention relates to a polymer electrolyte suitable for a fuel cell, a process for producing the same, and a fuel cell using the same.
2. Description of the Related Art
Due to recent attention to various environmental problems, a new energy technique has thus attracted special interests. It is such a fuel cell technique which is considered a promising new energy technique and it is anticipated that it will become one of the most important technology in the future. A polymer type fuel cell using a proton-conductive polymer as an electrolyte has attracted such special interest because of features such as good operation property at low temperatures, possibility of miniaturization and creating light, etc.
As a polymer electrolyte for a polymer type fuel cell, for example, Nafion (trademark of Du Pont Co.) as a ultrastrong acid group containing fluorinated polymer is known. However, Nafion is very expensive because it is a fluorinated polymer, and the control of water must be strictly followed because of its low water retention in the case of using it as a fuel cell. Furthermore, it is necessary when using a fluorine-containing compound to take the environment into consideration in respect to synthesis and disposal. Therefore, a polymer electrolyte of a non-fluorine proton-conductive material is desirable by today's market standard.
Some studies have already been made with respect to the non-fluorinated polymer-based polymeric proton-conductive material. In the 1950s, poly(styrenesulfonic acid) cation-exchange resins were examined. However, since the strength of a film which is the usual form at the time of using for a fuel cell was not enough, sufficient cell life could not be obtained.
A fuel cell using a sulfonated aromatic polyether ether ketone as the electrolyte has been researched. A detailed report on the synthesis of the sulfonated aromatic polyether ether ketone and its characteristics are disclosed in Polymer, page 1009, vol. 28 (1987). This report discloses that an aromatic polyether ether ketone (sometimes, referred to as PEEK), which is insoluble in organic solvent, becomes soluble in some organic solvent by highly sulfonating it, thereby facilitating film formation. The hydrophilic nature of these sulfonated PEEK has increased thus the water-solubility or a reduction in strength on water absorption occurred. When the water-solubility occurs, a fuel cell normally forms water as a by-product as a result of a reaction between fuel and oxygen, the sulfonated PEEK is therefore not suitable to use as an electrolyte for a fuel cell as it is. Japanese Patent Kokai Publication No. 6-93114 with respect to an electrolyte of sulfonated PEEK shows that an electrolyte having excellent strength can be obtained by introducing crosslinkable functional groups to a polymer and crosslinking the functional groups after film formation.
Polymeric Material Science and Engineering, 68, 122-123 (1993) and U.S. Pat. No. 5,271,813 disclose that sulfonated compounds of aromatic polyether sulfone can be used as aromatic polyether sulfone, etc.) can be utilized as an electrolyte of a device for electrolysis of water (UDEL P-1700 used as polyether sulfone is a polymer classified as polysulfone (sometimes, referred to as PSF)). However, there is no description about various physical properties such as primary structure or ion-exchange group equivalent weight of these sulfonated compounds of polymers. As PSF has the diphenylpropane unit in the repeating unit of the molecule which is easily sulfonated, it is difficult for the resulting sulfonated polysulfone not to dissolve in water because of its high water absorption.
Journal of Membrane Science, 83(1993) 211-220 discloses sulfonated compounds of PSF (UDEL P-1700) and PES. In the journal, it is described that the sulfonated PSF becomes completely water-soluble and the evaluation as an electrolyte cannot be performed. Moreover, about the sulfonated PES, although it is not water-soluble, introduction of crosslinking structure is proposed from the problem of its high water absorption.
These conventional techniques have problems such as expensive electrolyte, difficulty of structure controlling, lack of water resistance, insufficient strength, difficuly in production or mold processing, etc.